1. Field of the Invention
The present invention relates to a projection aligner for use in the process of manufacturing LSIs.
2. Description of the Related Art
FIG. 11 shows the optical system of a conventional projection aligner. Light emitted from a light source 11, such as a mercury lamp, passes through a condensing lens 12 and then irradiates a mask 13 on which a circuit pattern is formed. The light which has passed through the mask 13 projects the circuit pattern onto a wafer 15, whereby the
circuit pattern is exposed and then transferred.
Before the circuit pattern is exposed, the conditions under which a pattern is focused completely (hereinafter referred to as best focus conditions) must be determined. In a conventional exposure process, the best focus conditions are determined in the following manner. First, test patterns 16a to 16g as shown in FIG. 12 are successively exposed on the wafer 15 while the wafer 15 is moved along the optical axis at predetermined intervals. The wafer 15 is then developed. As shown in FIG. 13, patterns 17a to 17g corresponding to the test patterns 16a to 16g are formed on the wafer 15, and then observed with a microscope. Of the patterns 17a to 17g, the pattern having the best shape is found. The conditions under which such a pattern is formed are regarded as the best focus conditions. The circuit pattern is exposed under the best focus conditions. Of the patterns 17a to 17g shown in FIG. 13, for example, pattern 17c is determined as a pattern formed under the best focus conditions.
With such a conventional projection aligner, as described above, in order to determine the best focus conditions, before the circuit pattern is exposed, it is necessary to expose the test patterns 16a to 16g, to develop the wafer 15 and observe the patterns 17a to 17g with a microscope, such as an optical or electron microscope, thus requiring a great deal of trouble and time.